Combustion engines of the above mentioned type are known. They reduce fuel consumption and exhaust emissions, but it is difficult to control the air/fuel ratio in such an engine. U.S. Pat. No. 4,075,985 shows an example of a two-stroke engine where air ducts connect to the upper part of the engine's transfer ducts. Check valves are arranged at the connection between the ducts. A restriction valve is arranged in the air supply system to the transfer ducts. This is mechanically connected to the throttle valve of the carburetor of the engine, so that the two valves are following each other.
U.S. Pat. No. 5,425,346 shows an engine with a somewhat different design than that described above. In the '346 patent, channels are arranged in the piston of the engine which at specific piston positions are aligned with ducts arranged in the cylinder. Fresh air, as shown in FIG. 7, or exhaust gases can thereby be added to the upper part of the transfer ducts. This only happens at the specific piston positions where the ducts in the piston and the cylinder are aligned. This happens both when the piston moves downwards and when the piston moves upwards, but far away from the top dead center position. To avoid unwanted flow in the wrong direction in the latter case, check valves are arranged at the inlet to the upper part of the transfer ducts. In this respect it consequently corresponds to the previously mentioned patent. These type of check valves, usually called reed valves, have a number of disadvantages. They frequently have a tendency to come into resonant oscillations and can have difficulties coping with the high rotational speeds that many two-stroke engines can reach. Besides, it results in added cost and an increased number of engine components. Should such a valve break into smaller pieces, the pieces can enter into the engine and cause severe damages. The amount of fresh air added is, for the solution according to the '346 patent, varied by means of a variable inlet, i.e. an inlet that can be advanced or retarded in the work cycle. This is, however, a very complicated solution.
The international patent application WO98/57053 shows a few different embodiments of an engine where air is supplied to the transfer ducts via L-shaped or T-shaped recesses in the piston. Thus, there are no check valves. In all embodiments, the piston recess has, where it meets the respective transfer duct, a very limited height, which is essentially equal to the height of the actual transfer port. A consequence of this embodiment is that the passage for the air delivery through the piston to the transfer port is opened by the piston significantly later than is the passage for the air/fuel mixture to the crankcase. The period for the air supply is consequently significantly shorter than the period for the supply of air/fuel mixture, where the period can be counted as crank angle or be measured in time. This means that the amount of air that can be delivered to the transfer duct is significantly limited since the underpressure driving this additional air has significantly decreased because the inlet port has already been open during a certain period of time when the air supply is opened. This implies that both the period and the driving force for the air supply are small. Furthermore, the flow restriction in the L-shaped and the T-shaped ducts becomes relatively high. This is partly because the cross section of the duct is small close to the transfer port and partly because of the abrupt bend created by the L-shape or T-shape. In all, this contributes to reducing the amount of air that can be delivered to the transfer ducts which in turn reduces the possibilities to reduce the fuel consumption and the exhaust emissions by means of this arrangement.